Launched in November 2013, the Lumia 1520 is the oldest among the devices to be upgraded to Windows 10 Mobile. This is because it used a quad-core Snapdragon 800 chipset, rather than the dual-core Snapdragon S4 Plus that earlier Windows Phone 8 flagships used.

Keep in mind that getting Windows 10 Mobile on your device won't be as simple as checking for updates. First, you'll need to download the Upgrade Advisor app. Once you enable the upgrade through the app, head over to Settings -> Phone Update, and it will likely be downloading already.